In the picosecond ultrasonics technique, a short light pulse (the “pump”) is directed at the surface of the sample that is to be studied. When the light is absorbed, the temperature of the material near to the surface is raised slightly and a thermal stress is set up. This stress launches a strain pulse into the sample. The strain pulse is partially reflected at each interface that it encounters, thereby giving rise to reflected pulses that propagate back towards the sample surface. When one of these returning strain pulses reaches the sample surface, it causes a change in the optical reflectivity of the structure. This change is measured by means of a time-delayed light pulse (the “probe”). This technique makes it possible to perform ultrasonic experiments on a wide range of thin films and nanostructures. It is possible to measure the properties of a stack of thin films on a substrate and also to study films that have been laterally patterned.
The picosecond ultrasonics technique can be used to provide a non-destructive measurement of the thickness of thin films, particularly metal or other opaque films. The time, τ, for the sound pulse to make a round trip through a film is measured. This time equals 2d/v1, where d is the film thickness and v1 is the longitudinal sound velocity.
For most materials the sound velocity is well known. For materials for which v1 is not known from other measurements, it is often possible to make an estimate of v1 using the picosecond ultrasonics technique. For example, measurement of the reflection coefficient of the strain pulse at the interface between the film and the substrate, can be used to estimate the sound velocity. The reflection coefficient r12 of sound at an interface between a film 1 and a substrate 2 is given by the formula:
                              r          12                =                                            Z              1                        -                          Z              2                                                          Z              1                        +                          Z              2                                                          (        1        )            where Z1=ρ1v1 and Z2=ρ2v2, where ρ1 and ρ2 being the mass density of the film and substrate, and v1 and v2 their sound velocities. Thus, assuming that the densities and the sound velocity in the substrate, v2, are known, a measurement of r12 can be used to determine the sound velocity, v1, in the film.
Alternatively, measurement of the ultrasonic round trip time in one sample can be measured and the thickness of the sample can then be measured by scanning electron microscopy. This destroys that particular sample but provides a value of the sound velocity which can then be used to determine the thickness of other samples that have been prepared from the same material.
In addition to the determination of film thickness, the picosecond ultrasonic method can be used to determine other properties of thin film materials. These include, but are not limited to, the determination of grain size, grain orientation, adhesion between a film and a substrate, phase changes in the film material, electrical and thermal conductivity and electromigration.
With respect to grain size and orientation, the grain size of the sample results in an attenuation of the propagating sound pulses. The attenuation has a variation with frequency which results in a change in the shape of the sound pulse as it propagates. Thus, the shape of successive echoes of sound bouncing back and forth inside a thin film will be different and this change in shape gives information about the grain size of the film material. Further, in a typical picosecond ultrasonic measurement the detected change in optical reflectivity arises from (A) the propagating strain pulses, together with additional contributions from (B) the transient temperature change of the film in which the light is absorbed and (C) the sudden change in the distribution of the electrons that occurs immediately after the pump light pulse has been absorbed. These three components can be distinguished because each has a different and characteristic variation with time. From a comparison of the relative magnitudes of these three contributions it is possible to deduce the grain orientation of the sample.
With respect to the measurement of adhesion, when a film is well bonded to a substrate or to an adjacent film, the reflection of sound at the interface is given by the acoustic mismatch formula already given as Eq. (1). This reflection coefficient is independent of frequency. For a film that is poorly bonded the reflection coefficient will normally be higher and will vary with frequency. Usually, the reflection coefficient is higher at higher frequency. A variation of the reflection coefficient with frequency results in a change in the shape of the reflected sound pulse and this change in shape can be analyzed to give information about the adhesion.
Changes in the phase of the material making up a film can be detected in several ways. There will normally be a change in the sound velocity. The shape and size of the ultrasonic echoes will change. In addition, there will normally be changes in the components of the measured change in reflectivity arising from the transient temperature change and the change in the electron distribution (see (B) and (C) mentioned under grain orientation).
With respect to electrical and thermal conductivity, when the pump pulse is absorbed in the sample, energy is first given to the electrons close to the surface. These electrons diffuse rapidly into the interior of the material before they lose energy and transfer their energy to the phonons (thermal lattice vibrations). The distance over which this diffusion takes place affects the shape of the sound pulse that is generated. From a measurement of the shape of the sound pulse it is possible to make an estimate of the rate at which the electrons diffuse and from this to determine the electrical and thermal conductivity. For example, in the case of a sufficiently thin metal film of high conductivity, the electrons may diffuse throughout the thickness of the film before giving their energy to the lattice. In this case sound pulses are generated from both the front and the back of the film. The relative magnitudes of the front and back pulses is related to the conductivity.
Lastly, when a metal film carries current, electromigration can result in a change in the thickness of certain regions of the film with time. Thus, electromigration can be detected through a precision measurement of changes in the film thickness.
The above methods rely on an accurate measurement of the change in the optical reflectivity of the sample by means of an applied time-delayed probe light pulse, e.g., a measurement of the change in the intensity of the reflected probe pulse. Instead of measuring the change in the intensity of the reflected probe pulse, measurements can also be made of the change in the phase of the reflected probe pulse, the change in the polarization of the reflected probe pulse, the change in the direction of the reflected probe pulse and, in the case of samples that are partially transparent measurement of the change in the intensity of the transmitted probe pulse.
In sum, in the conventional picosecond ultrasonic technique the probe light is directed at the surface of the sample and the intensity, or other attribute (see above), of the reflected probe light is measured. Exemplary patents describing such conventional picosecond ultrasonic techniques and devices include U.S. Pat. Nos. 5,748,317 (characterizing thin film and interface characteristics using optical heat generator and detector); 5,748,318 (optical stress generator and detector); 5,959,735 (improved optical stress generator and detector); 5,864,393 (stress in thin films); 5,844,684 (mechanical properties of materials); 6,038,026 (grain size in thin films); 6,025,918 (electromigration); 6,317,216 (grain orientation); and 6,321,601 (characterizing laterally-patterned samples in an integrated circuit (IC)).